diff --git a/src/google/adk/tools/_gemini_schema_util.py b/src/google/adk/tools/_gemini_schema_util.py
new file mode 100644
index 0000000..92bf770
--- /dev/null
+++ b/src/google/adk/tools/_gemini_schema_util.py
@@ -0,0 +1,130 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+import re
+from typing import Any
+from typing import Optional
+
+from google.genai.types import JSONSchema
+from google.genai.types import Schema
+from pydantic import Field
+
+from ..utils.variant_utils import get_google_llm_variant
+
+
+class _ExtendedJSONSchema(JSONSchema):
+  property_ordering: Optional[list[str]] = Field(
+      default=None,
+      description="""Optional. The order of the properties. Not a standard field in open api spec. Only used to support the order of the properties.""",
+  )
+
+
+def _to_snake_case(text: str) -> str:
+  """Converts a string into snake_case.
+
+  Handles lowerCamelCase, UpperCamelCase, or space-separated case, acronyms
+  (e.g., "REST API") and consecutive uppercase letters correctly.  Also handles
+  mixed cases with and without spaces.
+
+  Examples:
+  ```
+  to_snake_case('camelCase') -> 'camel_case'
+  to_snake_case('UpperCamelCase') -> 'upper_camel_case'
+  to_snake_case('space separated') -> 'space_separated'
+  ```
+
+  Args:
+      text: The input string.
+
+  Returns:
+      The snake_case version of the string.
+  """
+
+  # Handle spaces and non-alphanumeric characters (replace with underscores)
+  text = re.sub(r"[^a-zA-Z0-9]+", "_", text)
+
+  # Insert underscores before uppercase letters (handling both CamelCases)
+  text = re.sub(r"([a-z0-9])([A-Z])", r"\1_\2", text)  # lowerCamelCase
+  text = re.sub(
+      r"([A-Z]+)([A-Z][a-z])", r"\1_\2", text
+  )  # UpperCamelCase and acronyms
+
+  # Convert to lowercase
+  text = text.lower()
+
+  # Remove consecutive underscores (clean up extra underscores)
+  text = re.sub(r"_+", "_", text)
+
+  # Remove leading and trailing underscores
+  text = text.strip("_")
+
+  return text
+
+
+def _sanitize_schema_formats_for_gemini(schema_node: Any) -> Any:
+  """Helper function to sanitize schema formats for Gemini compatibility"""
+  if isinstance(schema_node, dict):
+    new_node = {}
+    current_type = schema_node.get("type")
+
+    for key, value in schema_node.items():
+      key = _to_snake_case(key)
+
+      # special handle of format field
+      if key == "format":
+        current_format = value
+        format_to_keep = None
+        if current_format:
+          if current_type == "integer" or current_type == "number":
+            if current_format in ("int32", "int64"):
+              format_to_keep = current_format
+          elif current_type == "string":
+            # only 'enum' and 'date-time' are supported for STRING type"
+            if current_format in ("date-time", "enum"):
+              format_to_keep = current_format
+          # For any other type or unhandled format
+          # the 'format' key will be effectively removed for that node.
+          if format_to_keep:
+            new_node[key] = format_to_keep
+        continue
+      # don't change property name
+      if key == "properties":
+        new_node[key] = {
+            k: _sanitize_schema_formats_for_gemini(v) for k, v in value.items()
+        }
+        continue
+      # Recursively sanitize other parts of the schema
+      new_node[key] = _sanitize_schema_formats_for_gemini(value)
+    return new_node
+  elif isinstance(schema_node, list):
+    return [_sanitize_schema_formats_for_gemini(item) for item in schema_node]
+  else:
+    return schema_node
+
+
+def _to_gemini_schema(openapi_schema: dict[str, Any]) -> Schema:
+  """Converts an OpenAPI schema dictionary to a Gemini Schema object."""
+  if openapi_schema is None:
+    return None
+
+  if not isinstance(openapi_schema, dict):
+    raise TypeError("openapi_schema must be a dictionary")
+
+  openapi_schema = _sanitize_schema_formats_for_gemini(openapi_schema)
+  return Schema.from_json_schema(
+      json_schema=_ExtendedJSONSchema.model_validate(openapi_schema),
+      api_option=get_google_llm_variant(),
+  )
diff --git a/src/google/adk/tools/apihub_tool/apihub_toolset.py b/src/google/adk/tools/apihub_tool/apihub_toolset.py
index 62c183a..747650b 100644
--- a/src/google/adk/tools/apihub_tool/apihub_toolset.py
+++ b/src/google/adk/tools/apihub_tool/apihub_toolset.py
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
 
 from typing import List
 from typing import Optional
@@ -23,9 +24,9 @@ import yaml
 from ...agents.readonly_context import ReadonlyContext
 from ...auth.auth_credential import AuthCredential
 from ...auth.auth_schemes import AuthScheme
+from .._gemini_schema_util import _to_snake_case
 from ..base_toolset import BaseToolset
 from ..base_toolset import ToolPredicate
-from ..openapi_tool.common.common import to_snake_case
 from ..openapi_tool.openapi_spec_parser.openapi_toolset import OpenAPIToolset
 from ..openapi_tool.openapi_spec_parser.rest_api_tool import RestApiTool
 from .clients.apihub_client import APIHubClient
@@ -171,7 +172,7 @@ class APIHubToolset(BaseToolset):
     if not spec_dict:
       return
 
-    self.name = self.name or to_snake_case(
+    self.name = self.name or _to_snake_case(
         spec_dict.get('info', {}).get('title', 'unnamed')
     )
     self.description = self.description or spec_dict.get('info', {}).get(
diff --git a/src/google/adk/tools/application_integration_tool/integration_connector_tool.py b/src/google/adk/tools/application_integration_tool/integration_connector_tool.py
index 47084cf..b3bb32a 100644
--- a/src/google/adk/tools/application_integration_tool/integration_connector_tool.py
+++ b/src/google/adk/tools/application_integration_tool/integration_connector_tool.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 import logging
 from typing import Any
 from typing import Dict
@@ -24,8 +26,8 @@ from typing_extensions import override
 from .. import BaseTool
 from ...auth.auth_credential import AuthCredential
 from ...auth.auth_schemes import AuthScheme
+from .._gemini_schema_util import _to_gemini_schema
 from ..openapi_tool.openapi_spec_parser.rest_api_tool import RestApiTool
-from ..openapi_tool.openapi_spec_parser.rest_api_tool import to_gemini_schema
 from ..openapi_tool.openapi_spec_parser.tool_auth_handler import ToolAuthHandler
 from ..tool_context import ToolContext
 
@@ -128,7 +130,7 @@ class IntegrationConnectorTool(BaseTool):
       if field in schema_dict['required']:
         schema_dict['required'].remove(field)
 
-    parameters = to_gemini_schema(schema_dict)
+    parameters = _to_gemini_schema(schema_dict)
     function_decl = FunctionDeclaration(
         name=self.name, description=self.description, parameters=parameters
     )
diff --git a/src/google/adk/tools/mcp_tool/mcp_tool.py b/src/google/adk/tools/mcp_tool/mcp_tool.py
index 40fadfb..463202b 100644
--- a/src/google/adk/tools/mcp_tool/mcp_tool.py
+++ b/src/google/adk/tools/mcp_tool/mcp_tool.py
@@ -20,6 +20,7 @@ from typing import Optional
 from google.genai.types import FunctionDeclaration
 from typing_extensions import override
 
+from .._gemini_schema_util import _to_gemini_schema
 from .mcp_session_manager import MCPSessionManager
 from .mcp_session_manager import retry_on_closed_resource
 
@@ -42,7 +43,6 @@ except ImportError as e:
 from ...auth.auth_credential import AuthCredential
 from ...auth.auth_schemes import AuthScheme
 from ..base_tool import BaseTool
-from ..openapi_tool.openapi_spec_parser.rest_api_tool import to_gemini_schema
 from ..tool_context import ToolContext
 
 logger = logging.getLogger("google_adk." + __name__)
@@ -99,7 +99,7 @@ class MCPTool(BaseTool):
         FunctionDeclaration: The Gemini function declaration for the tool.
     """
     schema_dict = self._mcp_tool.inputSchema
-    parameters = to_gemini_schema(schema_dict)
+    parameters = _to_gemini_schema(schema_dict)
     function_decl = FunctionDeclaration(
         name=self.name, description=self.description, parameters=parameters
     )
diff --git a/src/google/adk/tools/openapi_tool/common/common.py b/src/google/adk/tools/openapi_tool/common/common.py
index 8aa7cc4..7187b1b 100644
--- a/src/google/adk/tools/openapi_tool/common/common.py
+++ b/src/google/adk/tools/openapi_tool/common/common.py
@@ -12,8 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 import keyword
-import re
 from typing import Any
 from typing import Dict
 from typing import List
@@ -26,47 +27,7 @@ from pydantic import BaseModel
 from pydantic import Field
 from pydantic import model_serializer
 
-
-def to_snake_case(text: str) -> str:
-  """Converts a string into snake_case.
-
-  Handles lowerCamelCase, UpperCamelCase, or space-separated case, acronyms
-  (e.g., "REST API") and consecutive uppercase letters correctly.  Also handles
-  mixed cases with and without spaces.
-
-  Examples:
-  ```
-  to_snake_case('camelCase') -> 'camel_case'
-  to_snake_case('UpperCamelCase') -> 'upper_camel_case'
-  to_snake_case('space separated') -> 'space_separated'
-  ```
-
-  Args:
-      text: The input string.
-
-  Returns:
-      The snake_case version of the string.
-  """
-
-  # Handle spaces and non-alphanumeric characters (replace with underscores)
-  text = re.sub(r'[^a-zA-Z0-9]+', '_', text)
-
-  # Insert underscores before uppercase letters (handling both CamelCases)
-  text = re.sub(r'([a-z0-9])([A-Z])', r'\1_\2', text)  # lowerCamelCase
-  text = re.sub(
-      r'([A-Z]+)([A-Z][a-z])', r'\1_\2', text
-  )  # UpperCamelCase and acronyms
-
-  # Convert to lowercase
-  text = text.lower()
-
-  # Remove consecutive underscores (clean up extra underscores)
-  text = re.sub(r'_+', '_', text)
-
-  # Remove leading and trailing underscores
-  text = text.strip('_')
-
-  return text
+from ..._gemini_schema_util import _to_snake_case
 
 
 def rename_python_keywords(s: str, prefix: str = 'param_') -> str:
@@ -106,7 +67,7 @@ class ApiParameter(BaseModel):
     self.py_name = (
         self.py_name
         if self.py_name
-        else rename_python_keywords(to_snake_case(self.original_name))
+        else rename_python_keywords(_to_snake_case(self.original_name))
     )
     if isinstance(self.param_schema, str):
       self.param_schema = Schema.model_validate_json(self.param_schema)
diff --git a/src/google/adk/tools/openapi_tool/openapi_spec_parser/__init__.py b/src/google/adk/tools/openapi_tool/openapi_spec_parser/__init__.py
index 68f877c..f743e74 100644
--- a/src/google/adk/tools/openapi_tool/openapi_spec_parser/__init__.py
+++ b/src/google/adk/tools/openapi_tool/openapi_spec_parser/__init__.py
@@ -20,7 +20,6 @@ from .operation_parser import OperationParser
 from .rest_api_tool import AuthPreparationState
 from .rest_api_tool import RestApiTool
 from .rest_api_tool import snake_to_lower_camel
-from .rest_api_tool import to_gemini_schema
 from .tool_auth_handler import ToolAuthHandler
 
 __all__ = [
@@ -30,7 +29,6 @@ __all__ = [
     'OpenAPIToolset',
     'OperationParser',
     'RestApiTool',
-    'to_gemini_schema',
     'snake_to_lower_camel',
     'AuthPreparationState',
     'ToolAuthHandler',
diff --git a/src/google/adk/tools/openapi_tool/openapi_spec_parser/openapi_spec_parser.py b/src/google/adk/tools/openapi_tool/openapi_spec_parser/openapi_spec_parser.py
index 9535953..ac86cd0 100644
--- a/src/google/adk/tools/openapi_tool/openapi_spec_parser/openapi_spec_parser.py
+++ b/src/google/adk/tools/openapi_tool/openapi_spec_parser/openapi_spec_parser.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 import copy
 from typing import Any
 from typing import Dict
@@ -23,8 +25,8 @@ from pydantic import BaseModel
 
 from ....auth.auth_credential import AuthCredential
 from ....auth.auth_schemes import AuthScheme
+from ..._gemini_schema_util import _to_snake_case
 from ..common.common import ApiParameter
-from ..common.common import to_snake_case
 from .operation_parser import OperationParser
 
 
@@ -112,7 +114,7 @@ class OpenApiSpecParser:
         # If operation ID is missing, assign an operation id based on path
         # and method
         if "operationId" not in operation_dict:
-          temp_id = to_snake_case(f"{path}_{method}")
+          temp_id = _to_snake_case(f"{path}_{method}")
           operation_dict["operationId"] = temp_id
 
         url = OperationEndpoint(base_url=base_url, path=path, method=method)
diff --git a/src/google/adk/tools/openapi_tool/openapi_spec_parser/operation_parser.py b/src/google/adk/tools/openapi_tool/openapi_spec_parser/operation_parser.py
index 73e8ed6..f7a577a 100644
--- a/src/google/adk/tools/openapi_tool/openapi_spec_parser/operation_parser.py
+++ b/src/google/adk/tools/openapi_tool/openapi_spec_parser/operation_parser.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 import inspect
 from textwrap import dedent
 from typing import Any
@@ -25,9 +27,9 @@ from fastapi.openapi.models import Operation
 from fastapi.openapi.models import Parameter
 from fastapi.openapi.models import Schema
 
+from ..._gemini_schema_util import _to_snake_case
 from ..common.common import ApiParameter
 from ..common.common import PydocHelper
-from ..common.common import to_snake_case
 
 
 class OperationParser:
@@ -189,7 +191,7 @@ class OperationParser:
     operation_id = self._operation.operationId
     if not operation_id:
       raise ValueError('Operation ID is missing')
-    return to_snake_case(operation_id)[:60]
+    return _to_snake_case(operation_id)[:60]
 
   def get_return_type_hint(self) -> str:
     """Returns the return type hint string (like 'str', 'int', etc.)."""
diff --git a/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py b/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
index 0491805..1e451fe 100644
--- a/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
+++ b/src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
@@ -12,45 +12,36 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 from typing import Any
 from typing import Dict
 from typing import List
 from typing import Literal
 from typing import Optional
-from typing import Sequence
 from typing import Tuple
 from typing import Union
 
 from fastapi.openapi.models import Operation
 from google.genai.types import FunctionDeclaration
-from google.genai.types import Schema
 import requests
 from typing_extensions import override
 
 from ....auth.auth_credential import AuthCredential
 from ....auth.auth_schemes import AuthScheme
-from ....tools.base_tool import BaseTool
+from ..._gemini_schema_util import _to_gemini_schema
+from ..._gemini_schema_util import _to_snake_case
+from ...base_tool import BaseTool
 from ...tool_context import ToolContext
 from ..auth.auth_helpers import credential_to_param
 from ..auth.auth_helpers import dict_to_auth_scheme
 from ..auth.credential_exchangers.auto_auth_credential_exchanger import AutoAuthCredentialExchanger
 from ..common.common import ApiParameter
-from ..common.common import to_snake_case
 from .openapi_spec_parser import OperationEndpoint
 from .openapi_spec_parser import ParsedOperation
 from .operation_parser import OperationParser
 from .tool_auth_handler import ToolAuthHandler
 
-# Not supported by the Gemini API
-_OPENAPI_SCHEMA_IGNORE_FIELDS = (
-    "title",
-    "default",
-    "format",
-    "additional_properties",
-    "ref",
-    "def",
-)
-
 
 def snake_to_lower_camel(snake_case_string: str):
   """Converts a snake_case string to a lower_camel_case string.
@@ -70,117 +61,6 @@ def snake_to_lower_camel(snake_case_string: str):
   ])
 
 
-# TODO: Switch to Gemini `from_json_schema` util when it is released
-# in Gemini SDK.
-def normalize_json_schema_type(
-    json_schema_type: Optional[Union[str, Sequence[str]]],
-) -> tuple[Optional[str], bool]:
-  """Converts a JSON Schema Type into Gemini Schema type.
-
-  Adopted and modified from Gemini SDK. This gets the first available schema
-  type from JSON Schema, and use it to mark Gemini schema type. If JSON Schema
-  contains a list of types, the first non null type is used.
-
-  Remove this after switching to Gemini `from_json_schema`.
-  """
-  if json_schema_type is None:
-    return None, False
-  if isinstance(json_schema_type, str):
-    if json_schema_type == "null":
-      return None, True
-    return json_schema_type, False
-
-  non_null_types = []
-  nullable = False
-  # If json schema type is an array, pick the first non null type.
-  for type_value in json_schema_type:
-    if type_value == "null":
-      nullable = True
-    else:
-      non_null_types.append(type_value)
-  non_null_type = non_null_types[0] if non_null_types else None
-  return non_null_type, nullable
-
-
-# TODO: Switch to Gemini `from_json_schema` util when it is released
-# in Gemini SDK.
-def to_gemini_schema(openapi_schema: Optional[Dict[str, Any]] = None) -> Schema:
-  """Converts an OpenAPI schema dictionary to a Gemini Schema object.
-
-  Args:
-      openapi_schema: The OpenAPI schema dictionary.
-
-  Returns:
-      A Pydantic Schema object.  Returns None if input is None.
-      Raises TypeError if input is not a dict.
-  """
-  if openapi_schema is None:
-    return None
-
-  if not isinstance(openapi_schema, dict):
-    raise TypeError("openapi_schema must be a dictionary")
-
-  pydantic_schema_data = {}
-
-  # Adding this to force adding a type to an empty dict
-  # This avoid "... one_of or any_of must specify a type" error
-  if not openapi_schema.get("type"):
-    openapi_schema["type"] = "object"
-
-  for key, value in openapi_schema.items():
-    snake_case_key = to_snake_case(key)
-    # Check if the snake_case_key exists in the Schema model's fields.
-    if snake_case_key in Schema.model_fields:
-      if snake_case_key in _OPENAPI_SCHEMA_IGNORE_FIELDS:
-        # Ignore these fields as Gemini backend doesn't recognize them, and will
-        # throw exception if they appear in the schema.
-        # Format: properties[expiration].format: only 'enum' and 'date-time' are
-        # supported for STRING type
-        continue
-      elif snake_case_key == "type":
-        schema_type, nullable = normalize_json_schema_type(
-            openapi_schema.get("type", None)
-        )
-        # Adding this to force adding a type to an empty dict
-        # This avoid "... one_of or any_of must specify a type" error
-        pydantic_schema_data["type"] = schema_type if schema_type else "object"
-        pydantic_schema_data["type"] = pydantic_schema_data["type"].upper()
-        if nullable:
-          pydantic_schema_data["nullable"] = True
-      elif snake_case_key == "properties" and isinstance(value, dict):
-        pydantic_schema_data[snake_case_key] = {
-            k: to_gemini_schema(v) for k, v in value.items()
-        }
-      elif snake_case_key == "items" and isinstance(value, dict):
-        pydantic_schema_data[snake_case_key] = to_gemini_schema(value)
-      elif snake_case_key == "any_of" and isinstance(value, list):
-        pydantic_schema_data[snake_case_key] = [
-            to_gemini_schema(item) for item in value
-        ]
-      # Important:  Handle cases where the OpenAPI schema might contain lists
-      # or other structures that need to be recursively processed.
-      elif isinstance(value, list) and snake_case_key not in (
-          "enum",
-          "required",
-          "property_ordering",
-      ):
-        new_list = []
-        for item in value:
-          if isinstance(item, dict):
-            new_list.append(to_gemini_schema(item))
-          else:
-            new_list.append(item)
-        pydantic_schema_data[snake_case_key] = new_list
-      elif isinstance(value, dict) and snake_case_key not in ("properties"):
-        # Handle dictionary which is neither properties or items
-        pydantic_schema_data[snake_case_key] = to_gemini_schema(value)
-      else:
-        # Simple value assignment (int, str, bool, etc.)
-        pydantic_schema_data[snake_case_key] = value
-
-  return Schema(**pydantic_schema_data)
-
-
 AuthPreparationState = Literal["pending", "done"]
 
 
@@ -273,7 +153,7 @@ class RestApiTool(BaseTool):
         parsed.operation, parsed.parameters, parsed.return_value
     )
 
-    tool_name = to_snake_case(operation_parser.get_function_name())
+    tool_name = _to_snake_case(operation_parser.get_function_name())
     generated = cls(
         name=tool_name,
         description=parsed.operation.description
@@ -306,7 +186,7 @@ class RestApiTool(BaseTool):
   def _get_declaration(self) -> FunctionDeclaration:
     """Returns the function declaration in the Gemini Schema format."""
     schema_dict = self._operation_parser.get_json_schema()
-    parameters = to_gemini_schema(schema_dict)
+    parameters = _to_gemini_schema(schema_dict)
     function_decl = FunctionDeclaration(
         name=self.name, description=self.description, parameters=parameters
     )
diff --git a/tests/unittests/tools/openapi_tool/common/test_common.py b/tests/unittests/tools/openapi_tool/common/test_common.py
index c188367..5dc8578 100644
--- a/tests/unittests/tools/openapi_tool/common/test_common.py
+++ b/tests/unittests/tools/openapi_tool/common/test_common.py
@@ -21,7 +21,6 @@ from fastapi.openapi.models import Schema
 from google.adk.tools.openapi_tool.common.common import ApiParameter
 from google.adk.tools.openapi_tool.common.common import PydocHelper
 from google.adk.tools.openapi_tool.common.common import rename_python_keywords
-from google.adk.tools.openapi_tool.common.common import to_snake_case
 from google.adk.tools.openapi_tool.common.common import TypeHintHelper
 import pytest
 
@@ -30,47 +29,6 @@ def dict_to_responses(input: Dict[str, Any]) -> Dict[str, Response]:
   return {k: Response.model_validate(input[k]) for k in input}
 
 
-class TestToSnakeCase:
-
-  @pytest.mark.parametrize(
-      'input_str, expected_output',
-      [
-          ('lowerCamelCase', 'lower_camel_case'),
-          ('UpperCamelCase', 'upper_camel_case'),
-          ('space separated', 'space_separated'),
-          ('REST API', 'rest_api'),
-          ('Mixed_CASE with_Spaces', 'mixed_case_with_spaces'),
-          ('__init__', 'init'),
-          ('APIKey', 'api_key'),
-          ('SomeLongURL', 'some_long_url'),
-          ('CONSTANT_CASE', 'constant_case'),
-          ('already_snake_case', 'already_snake_case'),
-          ('single', 'single'),
-          ('', ''),
-          ('  spaced  ', 'spaced'),
-          ('with123numbers', 'with123numbers'),
-          ('With_Mixed_123_and_SPACES', 'with_mixed_123_and_spaces'),
-          ('HTMLParser', 'html_parser'),
-          ('HTTPResponseCode', 'http_response_code'),
-          ('a_b_c', 'a_b_c'),
-          ('A_B_C', 'a_b_c'),
-          ('fromAtoB', 'from_ato_b'),
-          ('XMLHTTPRequest', 'xmlhttp_request'),
-          ('_leading', 'leading'),
-          ('trailing_', 'trailing'),
-          ('  leading_and_trailing_  ', 'leading_and_trailing'),
-          ('Multiple___Underscores', 'multiple_underscores'),
-          ('  spaces_and___underscores  ', 'spaces_and_underscores'),
-          ('  _mixed_Case  ', 'mixed_case'),
-          ('123Start', '123_start'),
-          ('End123', 'end123'),
-          ('Mid123dle', 'mid123dle'),
-      ],
-  )
-  def test_to_snake_case(self, input_str, expected_output):
-    assert to_snake_case(input_str) == expected_output
-
-
 class TestRenamePythonKeywords:
 
   @pytest.mark.parametrize(
diff --git a/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py b/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py
index adc6ba0..303dda6 100644
--- a/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py
+++ b/tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py
@@ -29,11 +29,9 @@ from google.adk.tools.openapi_tool.openapi_spec_parser.openapi_spec_parser impor
 from google.adk.tools.openapi_tool.openapi_spec_parser.operation_parser import OperationParser
 from google.adk.tools.openapi_tool.openapi_spec_parser.rest_api_tool import RestApiTool
 from google.adk.tools.openapi_tool.openapi_spec_parser.rest_api_tool import snake_to_lower_camel
-from google.adk.tools.openapi_tool.openapi_spec_parser.rest_api_tool import to_gemini_schema
 from google.adk.tools.tool_context import ToolContext
 from google.genai.types import FunctionDeclaration
 from google.genai.types import Schema
-from google.genai.types import Type
 import pytest
 
 
@@ -777,237 +775,6 @@ class TestRestApiTool:
     assert "empty_param" not in request_params["params"]
 
 
-class TestToGeminiSchema:
-
-  def test_to_gemini_schema_none(self):
-    assert to_gemini_schema(None) is None
-
-  def test_to_gemini_schema_not_dict(self):
-    with pytest.raises(TypeError, match="openapi_schema must be a dictionary"):
-      to_gemini_schema("not a dict")
-
-  def test_to_gemini_schema_empty_dict(self):
-    result = to_gemini_schema({})
-    assert isinstance(result, Schema)
-    assert result.type == Type.OBJECT
-    assert result.properties is None
-
-  def test_to_gemini_schema_dict_with_only_object_type(self):
-    result = to_gemini_schema({"type": "object"})
-    assert isinstance(result, Schema)
-    assert result.type == Type.OBJECT
-    assert result.properties is None
-
-  def test_to_gemini_schema_basic_types(self):
-    openapi_schema = {
-        "type": "object",
-        "properties": {
-            "name": {"type": "string"},
-            "age": {"type": "integer"},
-            "is_active": {"type": "boolean"},
-        },
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert isinstance(gemini_schema, Schema)
-    assert gemini_schema.type == Type.OBJECT
-    assert gemini_schema.properties["name"].type == Type.STRING
-    assert gemini_schema.properties["age"].type == Type.INTEGER
-    assert gemini_schema.properties["is_active"].type == Type.BOOLEAN
-
-  def test_to_gemini_schema_array_string_types(self):
-    openapi_schema = {
-        "type": "object",
-        "properties": {
-            "boolean_field": {"type": "boolean"},
-            "nonnullable_string": {"type": ["string"]},
-            "nullable_string": {"type": ["string", "null"]},
-            "nullable_number": {"type": ["null", "integer"]},
-            "object_nullable": {"type": "null"},
-            "multi_types_nullable": {"type": ["string", "null", "integer"]},
-            "empty_default_object": {},
-        },
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert isinstance(gemini_schema, Schema)
-    assert gemini_schema.type == Type.OBJECT
-    assert gemini_schema.properties["boolean_field"].type == Type.BOOLEAN
-
-    assert gemini_schema.properties["nonnullable_string"].type == Type.STRING
-    assert not gemini_schema.properties["nonnullable_string"].nullable
-
-    assert gemini_schema.properties["nullable_string"].type == Type.STRING
-    assert gemini_schema.properties["nullable_string"].nullable
-
-    assert gemini_schema.properties["nullable_number"].type == Type.INTEGER
-    assert gemini_schema.properties["nullable_number"].nullable
-
-    assert gemini_schema.properties["object_nullable"].type == Type.OBJECT
-    assert gemini_schema.properties["object_nullable"].nullable
-
-    assert gemini_schema.properties["multi_types_nullable"].type == Type.STRING
-    assert gemini_schema.properties["multi_types_nullable"].nullable
-
-    assert gemini_schema.properties["empty_default_object"].type == Type.OBJECT
-    assert not gemini_schema.properties["empty_default_object"].nullable
-
-  def test_to_gemini_schema_nested_objects(self):
-    openapi_schema = {
-        "type": "object",
-        "properties": {
-            "address": {
-                "type": "object",
-                "properties": {
-                    "street": {"type": "string"},
-                    "city": {"type": "string"},
-                },
-            }
-        },
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.properties["address"].type == Type.OBJECT
-    assert (
-        gemini_schema.properties["address"].properties["street"].type
-        == Type.STRING
-    )
-    assert (
-        gemini_schema.properties["address"].properties["city"].type
-        == Type.STRING
-    )
-
-  def test_to_gemini_schema_array(self):
-    openapi_schema = {
-        "type": "array",
-        "items": {"type": "string"},
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.type == Type.ARRAY
-    assert gemini_schema.items.type == Type.STRING
-
-  def test_to_gemini_schema_nested_array(self):
-    openapi_schema = {
-        "type": "array",
-        "items": {
-            "type": "object",
-            "properties": {"name": {"type": "string"}},
-        },
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.items.properties["name"].type == Type.STRING
-
-  def test_to_gemini_schema_any_of(self):
-    openapi_schema = {
-        "anyOf": [{"type": "string"}, {"type": "integer"}],
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert len(gemini_schema.any_of) == 2
-    assert gemini_schema.any_of[0].type == Type.STRING
-    assert gemini_schema.any_of[1].type == Type.INTEGER
-
-  def test_to_gemini_schema_general_list(self):
-    openapi_schema = {
-        "type": "array",
-        "properties": {
-            "list_field": {"type": "array", "items": {"type": "string"}},
-        },
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.properties["list_field"].type == Type.ARRAY
-    assert gemini_schema.properties["list_field"].items.type == Type.STRING
-
-  def test_to_gemini_schema_enum(self):
-    openapi_schema = {"type": "string", "enum": ["a", "b", "c"]}
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.enum == ["a", "b", "c"]
-
-  def test_to_gemini_schema_required(self):
-    openapi_schema = {
-        "type": "object",
-        "required": ["name"],
-        "properties": {"name": {"type": "string"}},
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.required == ["name"]
-
-  def test_to_gemini_schema_nested_dict(self):
-    openapi_schema = {
-        "type": "object",
-        "properties": {
-            "metadata": {
-                "type": "object",
-                "properties": {
-                    "key1": {"type": "object"},
-                    "key2": {"type": "string"},
-                },
-            }
-        },
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    # Since metadata is not properties nor item, it will call to_gemini_schema recursively.
-    assert isinstance(gemini_schema.properties["metadata"], Schema)
-    assert (
-        gemini_schema.properties["metadata"].type == Type.OBJECT
-    )  # add object type by default
-    assert len(gemini_schema.properties["metadata"].properties) == 2
-    assert (
-        gemini_schema.properties["metadata"].properties["key1"].type
-        == Type.OBJECT
-    )
-    assert (
-        gemini_schema.properties["metadata"].properties["key2"].type
-        == Type.STRING
-    )
-
-  def test_to_gemini_schema_ignore_title_default_format(self):
-    openapi_schema = {
-        "type": "string",
-        "title": "Test Title",
-        "default": "default_value",
-        "format": "date",
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-
-    assert gemini_schema.title is None
-    assert gemini_schema.default is None
-    assert gemini_schema.format is None
-
-  def test_to_gemini_schema_property_ordering(self):
-    openapi_schema = {
-        "type": "object",
-        "propertyOrdering": ["name", "age"],
-        "properties": {
-            "name": {"type": "string"},
-            "age": {"type": "integer"},
-        },
-    }
-
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.property_ordering == ["name", "age"]
-
-  def test_to_gemini_schema_converts_property_dict(self):
-    openapi_schema = {
-        "properties": {
-            "name": {"type": "string", "description": "The property key"},
-            "value": {"type": "string", "description": "The property value"},
-        },
-        "type": "object",
-        "description": "A single property entry in the Properties message.",
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.type == Type.OBJECT
-    assert gemini_schema.properties["name"].type == Type.STRING
-    assert gemini_schema.properties["value"].type == Type.STRING
-
-  def test_to_gemini_schema_remove_unrecognized_fields(self):
-    openapi_schema = {
-        "type": "string",
-        "description": "A single date string.",
-        "format": "date",
-    }
-    gemini_schema = to_gemini_schema(openapi_schema)
-    assert gemini_schema.type == Type.STRING
-    assert not gemini_schema.format
-
-
 def test_snake_to_lower_camel():
   assert snake_to_lower_camel("single") == "single"
   assert snake_to_lower_camel("two_words") == "twoWords"
diff --git a/tests/unittests/tools/test_gemini_schema_utils.py b/tests/unittests/tools/test_gemini_schema_utils.py
new file mode 100644
index 0000000..56aeca1
--- /dev/null
+++ b/tests/unittests/tools/test_gemini_schema_utils.py
@@ -0,0 +1,424 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from google.adk.tools._gemini_schema_util import _to_gemini_schema
+from google.adk.tools._gemini_schema_util import _to_snake_case
+from google.genai.types import Schema
+from google.genai.types import Type
+import pytest
+
+
+class TestToGeminiSchema:
+
+  def test_to_gemini_schema_none(self):
+    assert _to_gemini_schema(None) is None
+
+  def test_to_gemini_schema_not_dict(self):
+    with pytest.raises(TypeError, match="openapi_schema must be a dictionary"):
+      _to_gemini_schema("not a dict")
+
+  def test_to_gemini_schema_empty_dict(self):
+    result = _to_gemini_schema({})
+    assert isinstance(result, Schema)
+    assert result.type is None
+    assert result.properties is None
+
+  def test_to_gemini_schema_dict_with_only_object_type(self):
+    result = _to_gemini_schema({"type": "object"})
+    assert isinstance(result, Schema)
+    assert result.type == Type.OBJECT
+    assert result.properties is None
+
+  def test_to_gemini_schema_basic_types(self):
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "name": {"type": "string"},
+            "age": {"type": "integer"},
+            "is_active": {"type": "boolean"},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert isinstance(gemini_schema, Schema)
+    assert gemini_schema.type == Type.OBJECT
+    assert gemini_schema.properties["name"].type == Type.STRING
+    assert gemini_schema.properties["age"].type == Type.INTEGER
+    assert gemini_schema.properties["is_active"].type == Type.BOOLEAN
+
+  def test_to_gemini_schema_array_string_types(self):
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "boolean_field": {"type": "boolean"},
+            "nonnullable_string": {"type": ["string"]},
+            "nullable_string": {"type": ["string", "null"]},
+            "nullable_number": {"type": ["null", "integer"]},
+            "object_nullable": {"type": "null"},  # invalid
+            "multi_types_nullable": {
+                "type": ["string", "null", "integer"]
+            },  # invalid
+            "empty_default_object": {},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert isinstance(gemini_schema, Schema)
+    assert gemini_schema.type == Type.OBJECT
+    assert gemini_schema.properties["boolean_field"].type == Type.BOOLEAN
+
+    assert gemini_schema.properties["nonnullable_string"].type == Type.STRING
+    assert not gemini_schema.properties["nonnullable_string"].nullable
+
+    assert gemini_schema.properties["nullable_string"].type == Type.STRING
+    assert gemini_schema.properties["nullable_string"].nullable
+
+    assert gemini_schema.properties["nullable_number"].type == Type.INTEGER
+    assert gemini_schema.properties["nullable_number"].nullable
+
+    assert gemini_schema.properties["object_nullable"].type is None
+    assert gemini_schema.properties["object_nullable"].nullable
+
+    assert gemini_schema.properties["multi_types_nullable"].type is None
+    assert gemini_schema.properties["multi_types_nullable"].nullable
+
+    assert gemini_schema.properties["empty_default_object"].type is None
+    assert not gemini_schema.properties["empty_default_object"].nullable
+
+  def test_to_gemini_schema_nested_objects(self):
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "address": {
+                "type": "object",
+                "properties": {
+                    "street": {"type": "string"},
+                    "city": {"type": "string"},
+                },
+            }
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.properties["address"].type == Type.OBJECT
+    assert (
+        gemini_schema.properties["address"].properties["street"].type
+        == Type.STRING
+    )
+    assert (
+        gemini_schema.properties["address"].properties["city"].type
+        == Type.STRING
+    )
+
+  def test_to_gemini_schema_array(self):
+    openapi_schema = {
+        "type": "array",
+        "items": {"type": "string"},
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.type == Type.ARRAY
+    assert gemini_schema.items.type == Type.STRING
+
+  def test_to_gemini_schema_nested_array(self):
+    openapi_schema = {
+        "type": "array",
+        "items": {
+            "type": "object",
+            "properties": {"name": {"type": "string"}},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.items.properties["name"].type == Type.STRING
+
+  def test_to_gemini_schema_any_of(self):
+    openapi_schema = {
+        "anyOf": [{"type": "string"}, {"type": "integer"}],
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert len(gemini_schema.any_of) == 2
+    assert gemini_schema.any_of[0].type == Type.STRING
+    assert gemini_schema.any_of[1].type == Type.INTEGER
+
+  def test_to_gemini_schema_general_list(self):
+    openapi_schema = {
+        "type": "array",
+        "properties": {
+            "list_field": {"type": "array", "items": {"type": "string"}},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.properties["list_field"].type == Type.ARRAY
+    assert gemini_schema.properties["list_field"].items.type == Type.STRING
+
+  def test_to_gemini_schema_enum(self):
+    openapi_schema = {"type": "string", "enum": ["a", "b", "c"]}
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.enum == ["a", "b", "c"]
+
+  def test_to_gemini_schema_required(self):
+    openapi_schema = {
+        "type": "object",
+        "required": ["name"],
+        "properties": {"name": {"type": "string"}},
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.required == ["name"]
+
+  def test_to_gemini_schema_nested_dict(self):
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "metadata": {
+                "type": "object",
+                "properties": {
+                    "key1": {"type": "object"},
+                    "key2": {"type": "string"},
+                },
+            }
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    # Since metadata is not properties nor item, it will call to_gemini_schema recursively.
+    assert isinstance(gemini_schema.properties["metadata"], Schema)
+    assert (
+        gemini_schema.properties["metadata"].type == Type.OBJECT
+    )  # add object type by default
+    assert len(gemini_schema.properties["metadata"].properties) == 2
+    assert (
+        gemini_schema.properties["metadata"].properties["key1"].type
+        == Type.OBJECT
+    )
+    assert (
+        gemini_schema.properties["metadata"].properties["key2"].type
+        == Type.STRING
+    )
+
+  def test_to_gemini_schema_converts_property_dict(self):
+    openapi_schema = {
+        "properties": {
+            "name": {"type": "string", "description": "The property key"},
+            "value": {"type": "string", "description": "The property value"},
+        },
+        "type": "object",
+        "description": "A single property entry in the Properties message.",
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.type == Type.OBJECT
+    assert gemini_schema.properties["name"].type == Type.STRING
+    assert gemini_schema.properties["value"].type == Type.STRING
+
+  def test_to_gemini_schema_remove_unrecognized_fields(self):
+    openapi_schema = {
+        "type": "string",
+        "description": "A single date string.",
+        "format": "date",
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.type == Type.STRING
+    assert not gemini_schema.format
+
+  def test_sanitize_integer_formats(self):
+    """Test that int32 and int64 formats are preserved for integer types"""
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "int32_field": {"type": "integer", "format": "int32"},
+            "int64_field": {"type": "integer", "format": "int64"},
+            "invalid_int_format": {"type": "integer", "format": "unsigned"},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+
+    # int32 and int64 should be preserved
+    assert gemini_schema.properties["int32_field"].format == "int32"
+    assert gemini_schema.properties["int64_field"].format == "int64"
+    # Invalid format should be removed
+    assert gemini_schema.properties["invalid_int_format"].format is None
+
+  def test_sanitize_string_formats(self):
+    """Test that only date-time and enum formats are preserved for string types"""
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "datetime_field": {"type": "string", "format": "date-time"},
+            "enum_field": {
+                "type": "string",
+                "format": "enum",
+                "enum": ["a", "b"],
+            },
+            "date_field": {"type": "string", "format": "date"},
+            "email_field": {"type": "string", "format": "email"},
+            "byte_field": {"type": "string", "format": "byte"},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+
+    # date-time and enum should be preserved
+    assert gemini_schema.properties["datetime_field"].format == "date-time"
+    assert gemini_schema.properties["enum_field"].format == "enum"
+    # Other formats should be removed
+    assert gemini_schema.properties["date_field"].format is None
+    assert gemini_schema.properties["email_field"].format is None
+    assert gemini_schema.properties["byte_field"].format is None
+
+  def test_sanitize_number_formats(self):
+    """Test format handling for number types"""
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "float_field": {"type": "number", "format": "float"},
+            "double_field": {"type": "number", "format": "double"},
+            "int32_number": {"type": "number", "format": "int32"},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+
+    # float and double should be removed for number type
+    assert gemini_schema.properties["float_field"].format is None
+    assert gemini_schema.properties["double_field"].format is None
+    # int32 should be preserved even for number type
+    assert gemini_schema.properties["int32_number"].format == "int32"
+
+  def test_sanitize_nested_formats(self):
+    """Test format sanitization in nested structures"""
+    openapi_schema = {
+        "type": "object",
+        "properties": {
+            "nested": {
+                "type": "object",
+                "properties": {
+                    "date_str": {"type": "string", "format": "date"},
+                    "int_field": {"type": "integer", "format": "int64"},
+                },
+            },
+            "array_field": {
+                "type": "array",
+                "items": {"type": "string", "format": "uri"},
+            },
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+
+    # Check nested object
+    assert (
+        gemini_schema.properties["nested"].properties["date_str"].format is None
+    )
+    assert (
+        gemini_schema.properties["nested"].properties["int_field"].format
+        == "int64"
+    )
+    # Check array items
+    assert gemini_schema.properties["array_field"].items.format is None
+
+  def test_sanitize_anyof_formats(self):
+    """Test format sanitization in anyOf structures"""
+    openapi_schema = {
+        "anyOf": [
+            {"type": "string", "format": "email"},
+            {"type": "integer", "format": "int32"},
+            {"type": "string", "format": "date-time"},
+        ],
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+
+    # First anyOf should have format removed (email)
+    assert gemini_schema.any_of[0].format is None
+    # Second anyOf should preserve int32
+    assert gemini_schema.any_of[1].format == "int32"
+    # Third anyOf should preserve date-time
+    assert gemini_schema.any_of[2].format == "date-time"
+
+  def test_camel_case_to_snake_case_conversion(self):
+    """Test that camelCase keys are converted to snake_case"""
+    openapi_schema = {
+        "type": "object",
+        "minProperties": 1,
+        "maxProperties": 10,
+        "properties": {
+            "firstName": {"type": "string", "minLength": 1, "maxLength": 50},
+            "lastName": {"type": "string", "minLength": 1, "maxLength": 50},
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+
+    # Check snake_case conversion
+    assert gemini_schema.min_properties == 1
+    assert gemini_schema.max_properties == 10
+    assert gemini_schema.properties["firstName"].min_length == 1
+    assert gemini_schema.properties["firstName"].max_length == 50
+
+  def test_preserve_valid_formats_without_type(self):
+    """Test behavior when format is specified but type is missing"""
+    openapi_schema = {
+        "format": "date-time",  # No type specified
+        "properties": {
+            "field1": {"format": "int32"},  # No type
+        },
+    }
+    gemini_schema = _to_gemini_schema(openapi_schema)
+
+    # Format should be removed when type is not specified
+    assert gemini_schema.format is None
+    assert gemini_schema.properties["field1"].format is None
+
+  def test_to_gemini_schema_property_ordering(self):
+    openapi_schema = {
+        "type": "object",
+        "propertyOrdering": ["name", "age"],
+        "properties": {
+            "name": {"type": "string"},
+            "age": {"type": "integer"},
+        },
+    }
+
+    gemini_schema = _to_gemini_schema(openapi_schema)
+    assert gemini_schema.property_ordering == ["name", "age"]
+
+
+class TestToSnakeCase:
+
+  @pytest.mark.parametrize(
+      "input_str, expected_output",
+      [
+          ("lowerCamelCase", "lower_camel_case"),
+          ("UpperCamelCase", "upper_camel_case"),
+          ("space separated", "space_separated"),
+          ("REST API", "rest_api"),
+          ("Mixed_CASE with_Spaces", "mixed_case_with_spaces"),
+          ("__init__", "init"),
+          ("APIKey", "api_key"),
+          ("SomeLongURL", "some_long_url"),
+          ("CONSTANT_CASE", "constant_case"),
+          ("already_snake_case", "already_snake_case"),
+          ("single", "single"),
+          ("", ""),
+          ("  spaced  ", "spaced"),
+          ("with123numbers", "with123numbers"),
+          ("With_Mixed_123_and_SPACES", "with_mixed_123_and_spaces"),
+          ("HTMLParser", "html_parser"),
+          ("HTTPResponseCode", "http_response_code"),
+          ("a_b_c", "a_b_c"),
+          ("A_B_C", "a_b_c"),
+          ("fromAtoB", "from_ato_b"),
+          ("XMLHTTPRequest", "xmlhttp_request"),
+          ("_leading", "leading"),
+          ("trailing_", "trailing"),
+          ("  leading_and_trailing_  ", "leading_and_trailing"),
+          ("Multiple___Underscores", "multiple_underscores"),
+          ("  spaces_and___underscores  ", "spaces_and_underscores"),
+          ("  _mixed_Case  ", "mixed_case"),
+          ("123Start", "123_start"),
+          ("End123", "end123"),
+          ("Mid123dle", "mid123dle"),
+      ],
+  )
+  def test_to_snake_case(self, input_str, expected_output):
+    assert _to_snake_case(input_str) == expected_output